Process for raising the melting point of acetone-soluble copolymers



United States Patent PROCESS FOR RAISING THE MELTING POINT OFACETONE-SOLUBLE COPOLYMERS Marshall W. Duke, Midland, Mich., assignor toThe Dow .Chemical Company, Midland, Mich., a corporation of DelawareNoDrawing. Application July 11, 1956 Serial No. 597,090

'5 Claims. (Cl. 260-855 This invention relates to a novel process forraising the melting point of certain acetone-soluble copolymers. Moreparticularly it relates to such a process for raising the melting pointof acetone-soluble copolymers composed of vinylidene chloride andacrylonitrile.

In U. 5-. 2,681,325 are disclosed certain copolymers of vinylidenechloride and acrylonitrile which are soluble in acetone inconcentrations necessary for spinning continuous fibers and filamentsand for casting continuous films. Those copolymers contain from 30 to 55percent acrylonitrile and correspondingly from 70 to 45 percentvinylidene chloride. The acetone spinning dopes prepared with thosepolymers could be spun by either wet or dry spinning techniques, and thefibers so prepared were found to have in interesting family ofproperties for commercial use. These fibers had one serious drawback,however. They had a softening range or melting point that was so lowthat the fabrics woven from the fibers could not be ironed. Attemptswere made to raise the melting point of the fibers by increasing themolecular weight of the copolymer. Other attempts involved thepreparation of a more homogeneous copolymer by special polymerizationtechniques. Those attempts as well as all others either failed to raisethe melting point of the fibers or destroyed the acetone solubility ofthe copolymer. It would be desirable to have a process for preparingacetonesoluble copolymers composed of vinylidene chloride andacrylonitrile having higher melting points than the prior copolymers andthe provision of such a process is the principal object of thisinvention.

Other objects will be apparent as the description of the inventionproceeds.

It has now been found that the melting point of articles prepared fromacetone soluble copolymers of vinylidene chloride and acrylonitrile israised without destroying the acetone solubility by preparing an acetonedope comprising the copolymer, and an acetone soluble agent selectedfrom the group consisting of monoalkyl phosphites, dialkyl phosphites,monoalkyl phosphates, and dialkyl phosphates and fabricating the dopeinto an article which is substantially acetone free.

The copolymers useful in the process of this invention are thosedescribed in U. S. 2,681,325 as containing from 30 to 55 percent byweight of acrylonitrile and correspondingly from 70 to 45 percent byweight of vinylidene chloride. Optimum results are obtained when thecopolymer is one of 40 percent by Weight of acrylonitrile and 60 percentvinylidene chloride. For use in continuous articles such as fibers andfilms the copolymers should have a molecular weight which would give asolution viscosity of at least 1.5 centipoises and not to exceed 2.5centipoises when measured from a solution of 0.5 gram of the copolymerdissolved in 100 milliliters of dimethyl formamide at 25 C. Preferablythe viscosity as thus measured should be between 1.8 to 2.2 centipoises.

The acetone-soluble agent is selected from the group consisting ofmonoand dialkyl phosphites and the monoand dialkyl phosphates. Typicalexamples of useful agents are mono-n-butyl phosphate, mono-n-octylphosphate, mono-ethyl phosphite, dimethyl phosphite, and diethylphosphite. It is essenial that at least one and not more than two of theacidic hydrogens of the partially esterified acid of phosphorus bepresent. Phosphoric acid causes the copolymer to become insoluble and atrialkyl phosphate has no effect in raising the melting point of thefibers and films.

For casting films and spinning fibers it is desirable to have a solutionof from 10 to 20 percent by weight of the defined copolymer in acetone,and solutions of that concentration range may be easily prepared bystirring the polymer into the acetone at room temperature. Less stirringis needed when the acetone is warmed to 35 to 45 C. Followingdissolution of the copolymer in the acetone the phosphorus compound maybe dissolved in the dope. The order of dissolution of the copolymer andmodifier is not critical, and, if desired, the phosphorus compound maybe dissolved first, followed by the copolymer, or the dissolution of theingredients may be carried out simultaneously.

To achieve the stated objectives of the process of this invention ofraising the melting point of the article without sacrificing acetonesolubility, the phosphorus compound should be employed in an amount offrom 1 to 5 percent of the weight of the copolymer. When less than 1percent is used there is very little if any elevation of the meltingpoint, while the use of more than 5 percent can cause insolubility ofthe product.

Following preparation of the dope, a continuous article is fabricated bycasting a film or by wet or dry spinning fibers. The articles are thenair dried or subjected to moderate temperatures to remove any readilyvolatile matter, after which the articles are subjected to an elevatedtemperature for a short time to cause chemical reaction between thecopolymer and the phosphorus compound and to fuse or to sinter thearticle. For example, exposure of an article to a temperature of aboutC. for from 15 to 60 minutes will suffice. Temperatures of 150 to 200 C.are most useful.

The inventor believes that the elevation in melting point without lossof acetone solubility is due principally to a minor amount ofcross-linking of the copolymer chains. Fibers, fabrics and filmsproduced in accordance with this process are capable of being ironed atthe usual temperatures used for ironing synthetic fibers and the fabricsproduced therefrom. If desired, an acetone insoluble product may beprepared by using greater than the stated amounts of agent andsubjecting the formed article to elevated temperatures for prolongedperiods.

By way of illustration, several dopes were prepared from 240 parts byweight of anhydrous acetone, 57 parts by weight of a copolymer composedof 60 percent vinylidene chloride and 40 percent acrylonitrile, and 3parts by weight of several phosphorus containing agents. For comparativepurposes, one dope had no phosphorus containing agent, one usedortho-phosphoric acid, and one used triethyl phosphate. Each of thedopes was cast onto a stainless steel plate to a dry thickness of 0.0025inch. The wet cast films were air dried for 24 hours followed by an ovendrying for one hour at 55 C. to remove substantially all of the acetone.The dried films were then heated at 150 C. for up to 60 minutes afterwhich strips of the films inch wide were cut for determining an apparentmelting point and solubility. The apparent melting point is a meltingpoint under a low load, and often provides a sharp point for manypolymeric materials which soften over a range of temperatures and showno true melting point. The determination was made by weighing one end ofthe film strip equivalent to 1400 gms./cm. of cross-section. The otherend of the strip was clamped so that the weighted end hung free.

A variable resistance cylindrical heating element was placed so as tocontact about 0.5 inch of the film. The heating element was brought to175 C. before mounting the strip. The temperature was then raised untilthe film strip either melted and broke or until 300 C. was reached. Ifthe film had not melted at 300 C. it was considered to have beenadequately cross-linked.

Acetone solubility was determined by immersing the strips in acetone forone hour at room temperature. The strips were then examined visually andrated arbitrarily as soluble, partially soluble, and insoluble.

The results are listed in Table I.

It can be seen that the melting points of the films prepared inaccordance with this invention are at least 100 C. higher than the blankwhich contained no agent.

When the heat solubility of the film strips are compared it was foundthat the strips of this invention were at least as stable as the blank,i. e., they showed no more, and often less evidence of discolorationafter prolonged thermal exposure.

The invention has been illustrated by tests carried out A on filmstrips. It is equally applicable to the improvement in thermalproperties of filamentary products made from the acetone solutions ofthe copolymers and phosphorus compounds.

I claim:

1. A process for increasing the melting point of acetone solublecopolymers of vinylidene chloride without destroying acetone solubilitycomprising sequentially preparing an acetone solution of a copolymercomposed of from 30 to percent by weight of acrylonitrile andcorrespondingly from to 45 percent by weight of vinylidenc chloride andfrom 1 to 5 percent of the weight of said copolymcr of anacetone-soluble phosphorus containing agent selected from the groupconsisting of monoalkyl phosphites, dialkyl phosphites, mono-alkylphosphates and dialkyl phosphates, removing acetone from the solution,and finally subjecting the acetone-free composition to an elevatedtemperature until the melting point of the copolymer has been raised.

2. The process claimed in claim 1, wherein said acetone solutioncontains from 10 to 20 percentby weight of said copolymer.

3. The process claimed in claim 1, wherein said copolymer is composed of40 percent by weight of acrylonitrile and 60 percent by weight ofvinylidene chloride.

4. The process claimed in claim 1, wherein said elevated temperature isat least C.

5. The process claimed in claim 1, wherein the solution of copolymer andphosphorus compound is spun in filamentary form and the resultingproduct is freed from acetone and heated to a temperature of 150 to 200C.

References Cited in the file of this patent UNITED STATES PATENTS2,587,477 Hunter Feb. 26, 1952

1. A PROCESS FOR INCREASING THE MELTING POINT OF ACETONE SOLUBLECOPOLYMERS OF VINYLIDENE CHLORIDE WITHOUT DESTROYING ACETONE SOLUBILITYCOMPRISING SEQUENTIALLY PREPARING AN ACETONE SOLUTION OF A COPOLYMERCOMPOSED OF FROM 30 TO 55 PERCENT BY WEIGHT OF ACRYLONITRILE ANDCORRESPONDINGLY FROM 70 TO 45 PERCENT BY WEIGHT OF VINYLIDENE CHLORIDEAND FROM 1 TO 5 PERCENT OF THE WEIGHT OF SAID COPOLYMER OF ANACETONE-SOLUBLE PHOSPHORUS CONTAINING AGENT SELECTED FROM THE GROUPCONSISTING OF MONOALKYL PHOSPHITES, DIALKYL PHOSPHITES, MONO-ALKYLPHOSPHATES AND DIALKYL PHOSPHATES REMOVING ACETONE FROM THE SOLUTION ANDFINALLY SUBJECTING THE ACETONE-FREE COMPOSITION TO AN ELEVATEDTEMPERATURE UNTIL THE MELTING POINT OF THE COPOLYMER HAS BEEN RAISED.